From Superstring Derived Models

It was recently suggested that the reported anomalies in Rb and Rc can be interpreted as the effect of a heavy vector boson that couples to quarks and is universally decoupled from leptons. We examine how an extra gauge boson with this property can arise from superstring derived models. In a specific three generation model we show that the U(1)B−L symmetry combines with the horizontal flavor symmetries to form a universal leptophobic U(1) symmetry. In our model there is an enhancement of the color gauge group from twisted sectors. The enhancement occurs after the breaking of the unifying gauge symmetry by “Wilson lines”. The leptophobic U(1) symmetry then becomes a generator of the color SU(4) gauge group. We examine how similar symmetries may appear in other string models without the enhancement. We propose that if the current LEP anomalies persist it may be evidence for a certain class of un–unified superstring models. ∗ E-mail address: [email protected] † E-mail address: [email protected] Over the past few years LEP, SLC and the Tevatron experiments provided impressive confirmation of the Standard Model of particle physics and its gauge symmetry structure. Recently, however, there has been accumulating evidence at LEP that indicates deviation from the Standard Model predictions in the hadronic partial width at the Z–boson peak [1], which is commonly referred to as the Rb − Rc crisis. It was recently suggested by several groups [2] that the discrepancy between the predicted and measured values for the decay of the Z–boson to b and c–quarks could be explained by an additional heavy gauge boson that couples to quarks but is universally decoupled from leptons. If this interpretation of the data is correct it will have profound implication on attempts to understand the origin of the gauge and matter structure of the Standard Model. It might for example invalidate the traditional approaches to embed the Standard Model in a simple Grand Unified Group as those in their nature unify the interactions between quarks and leptons. In superstring models one also traditionally starts with an underlying unifying gauge group which is then broken to the Standard Model by using string and field theoretic symmetry breaking mechanisms. However, as the rank of the gauge group in string models is larger than those which are used in Grand Unified Theories, one can contemplate the possibility that a particular combination of the additional U(1) generators in the four dimensional Cartan subalgebra will combine to form a leptophobic U(1). Moreover, in a generic level one string model, massless states that produce the Standard Model representations, in general, must be charged with respect to additional U(1) or discrete symmetries. As the assignment of charges to the quarks and lepton depend on the specific compactification, it is difficult to envision how a generic compactification will produce the universal charge assignment which is needed. Furthermore, the charges of the quarks and leptons under the additional U(1) symmetries of the four dimensional gauge group depend on specific patterns of symmetry breaking in the string models. Therefore, if such a leptophobic U(1) symmetry is produced it might be a peculiar accident of a particular string model or perhaps of a class of string models. In this paper, we examine how a leptophobic U(1) can arise from superstring